Historically, cemented carbides were invented in the 1930's for use as tool bits, machining tools, and the like. Machining was the rate determining factor in the tooling industry, so it was important to obtain tooling which could withstand high speed machining to increase the productivity of the process.
Then, changes in automation saved time in the machining process by loading, unloading, moving and inspecting with machines and robots. This meant that the greatest amount of time was saved due to the elimination of human interaction. Now, again, the time spent on machining is getting renewed interest because it is once again a large percent of the time spent on a tool. As computers take over more and more of the operation, it becomes important to optimize our machining capabilities by increasing the speed of milling and cutting.
Developments are being made in the area of new tool materials that will be better than traditional materials in three ways. The first two ways are directed toward prevention of catastrophic failures, i.e. 1) fracture resistance, and 2) resistance to plastic deformation; while the third way, i.e. resistance to wearing, is directed to the gradual wearing down of the tool.
Fracture resistance, of course, refers to the resistance to pieces of the tool being severed, or fractured, while work is in progress. Measurements of transverse rupture strength, although not directly correlated to fracturing, seem to be the best indicator of fracture resistance. Those materials with high transverse rupture strength are normally less prone to fracture. The following is a listing of the transverse rupture strengths for the most commonly used cutting tool materials.
______________________________________ Transverse Rupture Strength Tool material GPa (psi) ______________________________________ pure Al.sub.2 O.sub.3 0.69 (100,000) Sialon 0.75 (125,000) CBN (Amborite .RTM.) 0.57 (113,000) Cemented WC 1.4-2.8 (200-400,000) Coated WC 1.0-2.1 (150-300,000) High speed steel 2.8+ (400,000+) ______________________________________ "Amborite .RTM."is a registered trademark of DeBeers, Johannesburg, South Africa.
It may be noted that the cemented WC has the highest transverse rupture strength. It may also be noted that high speed steel also has a very high value. This generally indicates that tool parts can be made into more complex geometries, including more positive rake angles and smaller edge-included angles. Another feature which may seem odd is that the coated WC has a lower value than the cemented WC. Although the coating may serve to extend the life of the carbide tool, it also acts as an area where cracks may initiate due to the bonding stresses at the interface between the substrate and the coating. These figures tend to dispel the misconception that the presence of a wear resistant coating relaxes the requirements on the substrate. Rather, the substrate is now required to have increased hot strength, and be more resistant to fracture.
Secondly, resistance to plastic deformation simply means that the tool material must have sufficient high temperature strength to maintain its shape at cutting temperatures. If the substrate begins to get "mushy" at the higher temperatures which are experienced during the cutting and milling operations, catastrophic failure will take place. Obviously, the melting point of the workpiece sets the temperature limit on the cutting temperature (assuming the melting point of the tool exceeds that of the workpiece). Below is a table which contrasts the softening point of various tooling materials to the melting point of common workpiece materials.
______________________________________ (Softening Workpiece Tool material Point) Material (Melting point) ______________________________________ High Speed Steel 873 K Aluminum 873-933 K Cemented WC 1373 K Superalloys 1573-1673 K Aluminum Oxide 1673 K Steel 1723-1773 K Cubic Boron Nitride 1773 K Titanium 1873-1923 K Diamond 1773 K Zirconium 2073-2123 K ______________________________________
Consequently, it can be seen that a wear resistant coating needs a substrate of greater hot strength to withstand the higher temperatures allowed by the wear resistant coating, without "deforming" and causing failure.
The tool failures which occur due to fracture or deformation are catastrophic and happen all at once. These types of failures disrupt a conventional factory, and cannot be tolerated in an automated machining system. If these catastrophic failures can be prevented, the goal of a tool manufacturer is to provide a material which is hard and tough enough to withstand wear for an extended period of time. Generally, at moderate cutting speeds, the life of the tool is determined by excessive rubbing of the tool on the workpiece surface. At higher speeds, crater wear is the main concern, with the crater deepening until edge failure results.
Many companies are trying new tool materials in order to increase fracture resistance, resistance to tool deformation, and resistance to wear. A significant number of companies are making cemented carbides, i.e. tungsten carbide (WC) powder mixed with cobalt metal, as a binder, pressed into the shape of the tool and sintered. A coating may also be preferred depending on the application. It has also been shown that various additives can enhance certain properties, and depending on the workpiece being cut or milled, individual properties may need to be enhanced. These properties include hardness, toughness, plastic deformation at high temperatures, crater resistance, and wear resistance. Solid solutions have been proposed, as well as metallic carbides, carbonitrides, and nitrides.
Prior patents have stated that tantalum (Ta) has been substituted into the base tungsten carbide composition in order to increase toughness, while chromium (Cr) improves corrosion resistance, and titanium (Ti) increases Vickers hardness values. Zirconium and hafnium appear to contribute to wear resistance, while other additives enhance other properties. U.S. Pat. No. 5,364,209, issued Nov. 15, 1994 to Kennametal Inc. of Latrobe, Pa. discloses a coated cutting tool with a substrate composed of a WC based solid solution cemented carbide material having at least 70 weight percent WC, and Ta 0-12 wt. %, Ti 0-10 wt. %, and a small amount of chromium, with a metallic binder of 8-12 wt. % Cobalt. A CVD and a PVD coating was deposited onto the substrate.
U.S. Pat. No. 5,330,553, issued Jul. 19, 1994 to Sandvik AB of Sandviken, Sweden discloses a sintered carbonitride alloy with highly alloyed binder phase containing hard constituents based on, in addition to Ti, W and/or Mo, one or more of the metals Zr, Hf, V, Nb, Ta or Cr in a 5-30 Wt % binder phase based on cobalt and/or nickel. The grain size of the hard constituents is stated to be generally less than 2 micrometers.
U.S. Pat. No. 5,288,676, issued Feb. 22, 1994 to Mitsubishi Materials Corporation, of Tokyo, Japan discloses a WC/Co matrix (grain size 0.2-1.5 micrometers) incorporating a (Ta: Ti)C solid solution (grain size 1.0-2.0 micrometers), along with unavoidable impurities of calcium, sulfur, aluminum, silicon and phosphorus. It is also stated at column 2, lines 51-55 that WC is not available with a grain size of less than 0.2 micrometers on an industrial basis. So, the disclosed WC powders are all larger than 0.2 micrometers.
U.S. Pat. No. 4,971,485, issued Nov. 20, 1990 to Sumitomo Electric Industries, Ltd. of Osaka, Japan discloses a drill having a shank portion made of cemented tungsten carbide having a restricted particle size of not more than 0.7 micrometers in order to attain sufficient strength against breaking. It also includes nitrogen in the cemented carbide in order to suppress grain growth of hard dispersed particles during sintering. Example 1 shows a composition of WC, (Ti:W)C solid solution, TaC, and NbC as the hard constituents, with Co as the binder phase.
It is clear, therefore, that it would be an advantage to have a tool and its corresponding material which would exhibit superior hardness, toughness, and wear resistance. The same advantages are clearly amenable to usage in tool inserts, tool substrates, and metal forming dies. Although prior art cemented carbides have been superior in wear resistance, they have been susceptible to breakage during use due to their inferiority in hardness and toughness. This is especially true when the requirements for high speed cutting and milling have placed their performances on the line, where the new machining apparatuses need tools which can achieve higher speed operations.